Currently, a pressure-resistant container for storing and transporting pressurized gas, e.g., Compressed Natural Gas (CNG), Compressed Hydrogen Gas (CHG) or low-temperature gas has been put into practical use. Conventionally, a metal pressure-resistant container having a high strength and a superior gas barrier property has been mainly used. However, because the metal pressure-resistant container has a heavy weight, it was difficult to apply the container to a fuel tank of an automobile and that of a spacecraft, which are required to have lighter weight. Thus, a pressure-resistant container having a relatively light weight, which is formed by surrounding the outer circumference of a cylindrical liner with an outer shell made of fiber reinforced resin composite has been suggested in recent years.
As a liner constituting such a pressure-resistant container, a metal liner having a superior gas barrier property has been suggested. As shown in FIG. 3, a metal liner is generally manufactured by carrying out a deep drawing processing for a metal plate 100 to form a container 110 and by welding a dome section 120 manufactured by another step with the opening section of this container 110.
The metal liner manufactured through the above step itself has a load resistance property having a certain level. Thus, it is possible to suppress the manufacturing cost by reducing the thickness of the composite outer shell. However, the liner is not expected to be significantly lightened. On the other hand, another technique for forming a very thin liner by performing a chemical etching for a metal liner manufactured through the above step, has been suggested. Although a liner can be expected to be significantly lightened, there was a problem that the manufacturing cost was increased.
In order to solve the problem of the metal liner, a technique for manufacturing a liner by carrying out a blow molding for thermoplastic resin, has been suggested. As shown in FIG. 4, the blow molding is that melted thermoplastic resin is extruded by an extruder 200 through a circular gap of a dice 210 to form a parison 300 (extrusion step), the parison 300 is placed between a pair of molds 220 to close the molds 220 (mold closing step), and gas is blown into the parison 300 in the closed molds to form the liner (blowing step). By adopting this blow molding, it is possible to significantly reduce time required for processing the liner and the manufacturing cost.
By the way, a liner constituting a pressure-resistant container must have a “gas barrier property”. Thus, in the liner, it is required to use thermoplastic resin having a superior gas barrier property. In recent years, “liquid crystal resin” has been suggested as thermoplastic resin having a superior gas barrier property. When liquid crystal resin is compared with thermoplastic resin (high density polyethylene) which has been currently used as material for a liner for a high pressure tank, liquid crystal resin has a gas barrier property about 400000 times or more higher than that of thermoplastic resin. A current high pressure tank using a high density polyethylene liner can be practically used under conditions of a CNG200 atm. However, while it is assumed that a tank pressure will increase in the future (e.g., 700 atm) or that hydrogen or helium having a small molar weight is used as storage gas, a technique for adapting liquid crystal resin to a liner has been developed (see, Patent Publication 1 for example).
Patent Publication 1: Japanese Laid-Open Publication No. H6-238738 (page 3, FIG. 1)